/Engine/Plugins/MovieScene/MovieRenderPipeline/Source/MovieRenderPipelineCore/Public/MovieRenderTileImage.h
The raw sizes should exactly match the tile size. The SubpixelOffset should be in the range of [0,1).AccumulateTile()then figures out which tiles it touches, and accumulates them.
void AccumulateTile ( const TArray64< float > & InRawData, int32 InRawSizeX, int32 InRawSizeY, int InRawChannel, FVector2D InSubpixelOffset )